1. Affirmative, yes, to convey agreeance.
2. Gives the subject a good characteristic, feeling or appearance.

In the case of (2) this is a contextual word, ie: the use of the words around it define the exact meaning of yep.

Examples:
"She is all yep" - The subject is in some way desirable, or has said or performed something desirable or agreeable to the speaker.
"Yep, that's a yep" - A double confirmation. Generally used after confirmation has been asked for on a particular subject.

Another word for yes, but often used in a text message to continue the conversion just a little bit more longer. Most of the time it gives the other person a chance to continue the conversation by a question

It is also a way to say I agree with what you said

Ya and any other form of yes works
Yes doesn't work, it is just awkward

Sarah: yeah, swimming is awesome
Max: yep
Sarah: Hey, are you going to that party tomorrow at brad's house

The word you use when you really want piss your girlfriend(s) off after they send you one of those messages where she is trying to spark a long in depth conversation about your relationship after you simply say "i am going out tonight."

An absolute yes that implies far more than a simple yes. Alternately, an affirmative response with minimal energy.

"You mean they actually did XYZ? Yep.... long silence. Translation: "Yes, they did, can you believe they had the gall!"
"Are you going to sit there all day? Yep...." Translation: "Yes, I most certainly am, and there is nothing you or anyone else can do about it."